16.4 Preparation of Alkenes | Hydrocarbons | Chemistry 11

Alkenes can be synthesized through various methods. This section focuses on common laboratory preparations: the dehydration of alcohols, dehydrohalogenation of halogenoalkanes, vicinal dehalogenation, and controlled hydrogenation of alkynes. These methods are primarily types of elimination reactions.

1. Dehydration of Alcohols

Dehydration is an elimination reaction that involves the removal of a water molecule ( $H_{2} O$ ) from an alcohol. This reaction is also classified as a β-elimination reaction, where a hydrogen atom from the β-carbon (the carbon adjacent to the carbon bearing the -OH group) and the hydroxyl group are removed, forming a double bond.

Key Features:

Reactant: An alcohol (e.g., ethanol).
Product: An alkene (e.g., ethene) and water.
Conditions: Requires a catalyst and heat.
Catalysts: The reaction is typically catalyzed by passing the alcohol vapors over hot aluminium oxide ( $A l_{2} O_{3}$ ), which also acts as a dehydrating agent. Other common catalysts include concentrated sulfuric acid ( $H_{2} S O_{4}$ ) and phosphoric acid ( $H_{3} P O_{4}$ ).

Ease of Dehydration: The ease of dehydration follows the order: Tertiary (3°) > Secondary (2°) > Primary (1°). Tertiary alcohols dehydrate most readily because they form more stable carbocation intermediates.

General Reaction:

$Alcohol Catalyst Heat Alkene + H_{2} O$

Example: Preparation of Ethene from Ethanol

When ethanol vapor is passed over hot aluminium oxide, it dehydrates to form ethene and water. Note that temperature matters; at $17 0^{\circ} C$ with $H_{2} S O_{4}$ , ethene is the major product.

$Ethanol C H_{3} C H_{2} O H A l_{2} O_{3} H e a t Ethene C H_{2} = C H_{2} + H_{2} O$

2. Dehydrohalogenation of Halogenoalkanes

Dehydrohalogenation is an elimination reaction involving the removal of a hydrogen atom and a halogen atom from adjacent carbon atoms in a halogenoalkane. This is another example of a β-elimination reaction.

Key Features:

Reactant: A halogenoalkane (e.g., bromoethane).
Reagent: A strong base dissolved in ethanol, such as hot ethanolic potassium hydroxide ( $K O H$ ). The alcoholic medium favors elimination over substitution.
Product: An alkene, a salt, and water.

General Reaction:

$Halogenoalkane + Base Ethanol Heat Alkene + Salt + H_{2} O$

Example: Preparation of Ethene from Bromoethane

Ethene can be prepared by heating bromoethane with a hot, ethanolic solution of potassium hydroxide.

$Bromoethane C H_{3} C H_{2} B r + K O H Ethanol Heat Ethene C H_{2} = C H_{2} + K B r + H_{2} O$

3. Dehalogenation of Vicinal Dihalides

Vicinal dihalides are compounds where two halogen atoms are attached to adjacent carbon atoms. When treated with Zinc dust in an anhydrous solvent like methanol or acetic acid, they undergo dehalogenation to form an alkene.

Reaction:

$Vicinal dihalide R - C H X - C H X - R + Z n \to R - C H = C H - R + Z n X_{2}$

4. Controlled Hydrogenation of Alkynes

Alkenes can be prepared by the partial reduction of alkynes. The stereochemistry of the product depends on the reducing agent used:

Cis-alkenes: Produced using hydrogen gas with Lindlar's catalyst ( $P d / B a S O_{4}$ poisoned with quinoline).
Trans-alkenes: Produced using Sodium in liquid ammonia ( $N a / liq. N H_{3}$ ).

Concept Assessment Exercise 16.2

Prepare but-1-ene by the following preparatory methods.

i. Dehydration of an alcohol ii. Dehydrohalogenation of a halogenoalkane

Solution

i. Preparation of But-1-ene by Dehydration of an Alcohol

To produce but-1-ene ( $C H_{2} = C H C H_{2} C H_{3}$ ), we must start with an alcohol where the -OH group is on carbon 1, which is butan-1-ol.

Reaction:

$Butan-1-ol C H_{3} C H_{2} C H_{2} C H_{2} O H conc. H_{2} S O_{4} Heat But-1-ene C H_{2} = C H C H_{2} C H_{3} + H_{2} O$

ii. Preparation of But-1-ene by Dehydrohalogenation of a Halogenoalkane

To produce but-1-ene, we must start with a halogenoalkane where the halogen is on carbon 1, such as 1-bromobutane.

Reaction:

$1-Bromobutane C H_{3} C H_{2} C H_{2} C H_{2} B r + K O H Ethanol Heat But-1-ene C H_{2} = C H C H_{2} C H_{3} + K B r + H_{2} O$

1. Dehydration of Alcohols

Key Features:

Reactant: An alcohol (e.g., ethanol).
Product: An alkene (e.g., ethene) and water.
Conditions: Requires a catalyst and heat.
Catalysts: The reaction is typically catalyzed by passing the alcohol vapors over hot aluminium oxide ( $A l_{2} O_{3}$ ), which also acts as a dehydrating agent. Other common catalysts include concentrated sulfuric acid ( $H_{2} S O_{4}$ ) and phosphoric acid ( $H_{3} P O_{4}$ ).

General Reaction:

$Alcohol Catalyst Heat Alkene + H_{2} O$

Example: Preparation of Ethene from Ethanol

When ethanol vapor is passed over hot aluminium oxide, it dehydrates to form ethene and water. Note that temperature matters; at $17 0^{\circ} C$ with $H_{2} S O_{4}$ , ethene is the major product.

$Ethanol C H_{3} C H_{2} O H A l_{2} O_{3} H e a t Ethene C H_{2} = C H_{2} + H_{2} O$

2. Dehydrohalogenation of Halogenoalkanes

Key Features:

Reactant: A halogenoalkane (e.g., bromoethane).
Reagent: A strong base dissolved in ethanol, such as hot ethanolic potassium hydroxide ( $K O H$ ). The alcoholic medium favors elimination over substitution.
Product: An alkene, a salt, and water.

General Reaction:

$Halogenoalkane + Base Ethanol Heat Alkene + Salt + H_{2} O$

Example: Preparation of Ethene from Bromoethane

Ethene can be prepared by heating bromoethane with a hot, ethanolic solution of potassium hydroxide.

$Bromoethane C H_{3} C H_{2} B r + K O H Ethanol Heat Ethene C H_{2} = C H_{2} + K B r + H_{2} O$

3. Dehalogenation of Vicinal Dihalides

Reaction:

$Vicinal dihalide R - C H X - C H X - R + Z n \to R - C H = C H - R + Z n X_{2}$

4. Controlled Hydrogenation of Alkynes

Alkenes can be prepared by the partial reduction of alkynes. The stereochemistry of the product depends on the reducing agent used:

Cis-alkenes: Produced using hydrogen gas with Lindlar's catalyst ( $P d / B a S O_{4}$ poisoned with quinoline).
Trans-alkenes: Produced using Sodium in liquid ammonia ( $N a / liq. N H_{3}$ ).

Concept Assessment Exercise 16.2

Prepare but-1-ene by the following preparatory methods.

i. Dehydration of an alcohol ii. Dehydrohalogenation of a halogenoalkane

Solution

i. Preparation of But-1-ene by Dehydration of an Alcohol

To produce but-1-ene ( $C H_{2} = C H C H_{2} C H_{3}$ ), we must start with an alcohol where the -OH group is on carbon 1, which is butan-1-ol.

Reaction:

$Butan-1-ol C H_{3} C H_{2} C H_{2} C H_{2} O H conc. H_{2} S O_{4} Heat But-1-ene C H_{2} = C H C H_{2} C H_{3} + H_{2} O$

ii. Preparation of But-1-ene by Dehydrohalogenation of a Halogenoalkane

To produce but-1-ene, we must start with a halogenoalkane where the halogen is on carbon 1, such as 1-bromobutane.

Reaction:

$1-Bromobutane C H_{3} C H_{2} C H_{2} C H_{2} B r + K O H Ethanol Heat But-1-ene C H_{2} = C H C H_{2} C H_{3} + K B r + H_{2} O$